Many implementations provide OES_depth_texture behavior if the
texture was specified with a non-sized format (e.g. GL_DEPTH_COMPONENT).
This change implements that behavior for Vulkan and adds a couple of
tests to verify it.

Bug: angleproject:3890
Change-Id: I005b1eaa30db033f7d78a5cf2236aab7f442b7f5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1764301
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>